Amplitude and phase modulation using dual digital delay vectors

ABSTRACT

The present invention provides a means to implement amplitude and phase modulation digitally and directly at an RF frequency that benefits from high output power using non-linear amplifiers. This is accomplished by the combination of two constant amplitude phase varying vectors. A reference oscillator produces a carrier signal, which is supplied to two digital delay lines composed of a sequence of delay banks. The delay lines are controlled by lookup tables that are updated by the vector control circuit used to determine the delay of each digital delay line. The delay of the lines are set in such a way as to produce two vectors with the desired phase shift that, when summed together, produce a vector with the desired phase and amplitude characteristics.

This application claims priority under 35 U.S.C. 119 from ProvisionalApplication Ser. No. 60/525,117 filed Nov. 28, 2003.

This application is related to an application filed simultaneously withthis application by the same inventors and entitled “Modulation usingdiscrete amplitude adjustment and dual digital delay lines”, thedisclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates generally to telecommunication systems. Thepresent invention relates specifically to data transmission using analogsignals, more specifically, to a unique method for providing amplitudeand phase modulation of a signal using dual digital delay lines.

BACKGROUND OF THE INVENTION

The following references may be relevant to the present invention:

-   -   U.S. Pat. No. 5,329,259—Stengel, “Efficient Amplitude/Phase        Modulation Amplifier”    -   U.S. Pat. No. 5,612,651—Chethik, “Modulating Array QAM        Transmitter”    -   U.S. Pat. No. 5,659,272—Linguet, “Amplitude Modulation Method        and Apparatus using Two Phase Modulated Signals”    -   U.S. Pat. No. 5,852,389—Kumar, “Direct QAM Modulator”    -   U.S. Pat. No. 5,867,071—Chethik, “High Power Transmitter        Employing a high Power QAM Modulator”    -   U.S. Pat. No. 6,147,553—Kolanek, “Amplification Using Amplitude        Reconstruction of Amplitude and/or Angle Modulated Carrier”    -   U.S. Pat. No. 6,160,856—Gershon, “System For Providing Amplitude        and Phase Modulation of Line Signals Using Delay Lines”    -   U.S. Pat. No. 6,313,703—Wright et al., “Use of Antiphase Signals        For Predistortion Training Within An Amplifier System”    -   U.S. Pat. No. 6,366,177—McCune, “High-Efficiency Power        Modulators”

With the ever increasing demand for the high speed transfer ofinformation digital systems are becoming very common. In its simplestform the modern telecommunication system requires circuits formodulation, frequency conversion, transmission and detection.

The basis for signal transmission is a continuous time varyingconstant-frequency signal known as a carrier. The carrier signal can berepresented as S(t)=A cos(2ζft+σ), where f is the frequency, A is theamplitude, and σ is the phase of the signal. S(t) is a deterministicsignal, and alone carries no useful information. However, informationcould be encoded on S(t) if one or more of the following characteristicsof the carrier were altered: amplitude, frequency or phase. In essencemodulation is the process of encoding an information source onto ahigh-frequency, carrier signal S(t).

Bandpass digital systems can be divided into two main categories; binarydigital systems or multilevel digital systems. Binary digital systemsare limited in that they can only represent a one bit symbol (0 or 1) atany given time. The most common binary bandpass signal techniques areAmplitude Shift Keying (ASK), Phase Shift Keying (PSK), and FrequencyShift Keying (FSK). For example, a binary digital system using ASK mighthave a signal range from 0 to 3 Volts. Any value less than 1.5 Voltswould represent a digital 0 and anything greater than 1.5 Volts wouldrepresent a digital 1. Alternatively, FSK would use two differentfrequencies and PSK would use two different phases to represent adigital 0 or 1. However, binary digital systems are not as practical asmultilevel systems since digital transmission is notoriously wasteful ofRF bandwidth, and regulatory authorities usually require a minimumbandwidth efficiency.

With multilevel digital systems, inputs with more than two modulationlevels are used. In cases like this multiple bits can be sent with eachsymbol, increasing the speed and efficiency. In keeping with theprevious example of an amplitude modulated signal with a range from 0 to3 Volts, the signal amplitude could be broken into 4 distinct points;0.75V could correspond to binary 00, 1.5V corresponds to 01, 2.25Vcorresponds to 10, and 3V corresponds to binary 11. In this case eachsymbol represents a two bit binary number. Alternatively, suchtransformations can be implemented by adjusting the phase or frequencyof the carrier.

More advanced techniques for a multilevel digital system would include acombination of amplitude and phase modulation of a carrier signal. Inthis case a single multi-bit symbol could be represented by a signalwith a certain phase and amplitude. Each symbol of digital data could bedefined as a vector with a specified amplitude and angle and visualizedon a polar axis. In one of its simplest forms a three bit digital symbolcould be represented by two distinct amplitudes and four distinctphases.

There are various common modulation techniques which require theamplitude and phase adjustment of a carrier signal. Solutions to thesemodulation techniques are typically built in either analog or digitalcircuitry. One such solution which is shown and described hereinafterwill be recognized by those skilled in the art as a IQ modulator. Due toits requirements for digital to analog conversion and linear poweramplification before transmission, modulators of this form typicallyconsume lots of power.

SUMMARY OF THE INVENTION

It is one object of the present invention to provide an apparatus foramplitude and phase modulation of a signal.

According to the invention there is provided an apparatus for amplitudeand phase modulation of a signal comprising:

-   -   a reference pulse oscillator arranged to provide a signal in the        form of a series of input pulses;    -   an input for input modulating data including desired amplitude        and phase modulation;    -   a vector logic circuit responsive to the input modulating data;    -   two digital delay lines each coupled to said reference        oscillator and each having multiple delay cells for selectively        delaying respective pulses of said signal;    -   two lookup tables each of which contains information for        controlling the delay cells of a respective one of the delay        lines so as to control an overall delay of the respective one of        the digital delay lines so as to generate therefrom a component        vector which is dependent upon the input modulating data;    -   and a summer that is coupled to the two digital delay lines and        arranged to combine together the component vectors from both of        the delay lines to provide an output vector.

Preferably said vector logic circuit utilizes the desired amplitude andphase modulation to determine the phase of the two fixed magnitudecomponent vectors.

Preferably said component vectors are assumed to have the same magnitudeand be equidistant, radially, from the resultant vector.

Preferably the formula ±Cos⁻¹[r/(2V)] governs the phase offset of thecomponent vectors from the desired output phase, where, in the governingformula, r represents the desired output magnitude and V is themagnitude of the component vectors.

Preferably said vector logic circuit compensates for the special caseswhere the phase of the leading or trailing vectors cross the 360°barrier, where compensation is accomplished by either adding orsubtracting 2π from the absolute phase of the vector.

Preferably said vector logic circuit converts the phase information intoan equivalent delay.

Preferably said vector logic circuit updates lookup tables with theinformation required to reproduce the required delay.

Preferably said delay lines contain a finite number of sequential orparallel delay cells capable of covering 360° of phase.

Preferably said delay cells have equivalent or weighted delay periods.

Preferably said delay lines contain a finite number of additional delaycells for the purpose of compensation in the range of the finestresolution step.

Preferably said delay cells contain a feedback edge detector whereupondetection of a falling edge enables the delay cell to confirm its nextstatus from a lookup table.

Preferably said lookup tables contain the information required toreproduce a specified delay.

Preferably said tables are directly referenced by the digital delaylines in order to control which delay cells are enabled at a given time.

Preferably said tables contain redundant registers which contain bothdelay and compensation information.

Preferably said summer is coupled to the two digital delay lines for thepurpose of combining two constant amplitude component vectors into aresultant vector containing a desired amplitude and phase.

Preferably said reference pulses are a high power pulse train.

The invention may provide one or more of the following advantages:

Digital data is converted into an analog signal without the use ofdigital to analog converters.

Digital data is converted into an analog signal which requires minimalamplification before transmission.

Digital data is converted into an analog signal which uses non-linearamplifiers.

It removes all digital to analog converters (DACs) from the modulationprocess.

It also provides a novel method for amplitude and phase modulation whichdoes not require linear amplification.

Removal of the DACs and linear amplifier, results in a significant powerreduction compared to conventional techniques.

The previously stated advantages are achieved, in part, by providing anamplitude and phase modulated system that produces two high powerconstant amplitude phase modulated vectors that, when summed together,will produce the desired amplitude and phase-modulated signal. In orderto facilitate this action, an input reference pulse is fed into twodigital delay lines (DDL) containing a specified number (N) of delayblocks. Unlike typical IQ modulator techniques, the reference signal,that is fed to the DDLs, does not have to be scaled back to maintainlinearity. Each delay line is controlled by a lookup table, whichcontains the required delay to shift the input reference pulse to thedesired phase. The phases of the two vectors are chosen by the vectorlogic block. The vector logic block updates the lookup tables for eachdelay line, thus establishing the phase of each vector. The phases ofthe vectors are chosen in such a way that when summed together theyproduce a resulting vector that contains both the desired phase andamplitude modulation.

Although the invention has general application in the field of signalmodulation, the most direct use of the method described in the inventionis the realization of a transmitter that converts digital data into anamplitude and phase modulated signal to be transmitted over acommunications line. In this case, the vector produced by the inventionrepresents a binary symbol. The number of bits in the symbol aredetermined by the encoding technique implemented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a prior art of IQ modulator.

FIG. 2 is a schematic block diagram of one embodiment of an apparatusaccording to the present invention.

FIG. 3 is a graphical representation of the vector math for theembodiment of FIG. 2.

FIG. 4 is a block diagram of the lookup table for the embodiment of FIG.2.

DETAILED DESCRIPTION

The present invention synthesizes a vector with the desired amplitudeand phase using two fixed magnitude vectors that have dynamicallycontrolled phases. FIG. 2 illustrates a block diagram of an embodimentof the invention. The apparatus consists of five major blocks; inputpulses 200, a vector logic circuit 201, two digital delay lines 202 aand 202 b, two lookup tables 203 a and 203 b, and a signal combiner 204.

The vector logic circuit 201 is supplied with digital data correspondingto the desired magnitude and phase of the output vector. Once the datahas been received the logic circuit determines the phase of the twovectors needed to generate the desired output vector. The vector logiccircuit 201 determines the phase of each vector by using the followingassumptions:

Both vectors will have the same magnitude.

Each vector will be equidistant, radially, from the resultant vector.

Having defined the vectors in the above manner the vector logic circuit201 can determine the phase of each vector. If the desired output vector300 (FIG. 3) has a magnitude r and phase θ the required angle ofrotation away from θ would be equal to Φ=Cos⁻¹[r/(2V)], where V is themagnitude of the each vector 301. The absolute phase of the leadingvector would be θ+Φ, while the absolute phase of the trailing vectorwould be θ−Φ. Special consideration must be taken when the leading ortrailing vector crosses over the 2π or 360° barrier. In such cases 2π iseither added to, or subtracted from, the absolute phase of the vectordepending upon whether it is the leading or trailing vector that hascrossed the bound. FIG. 3 shows a graphical example of the vector math.

Once the phase of both vectors required to reproduce the desired outputmagnitude and phase is determined, the vector logic circuit 201 convertsthe phase to a required delay time and updates the lookup tables 203 aand 203 b. Each table is used to select the delay cells required by thedelay lines 202 to synthesize the desired phase. The tables must beupdated no less than twice the speed of the symbol rate. Lookup table203 a contains the delay information for the vector A, while 203 bcontains the information for vector B. The preferred implementation ofthe invention would also include redundant blocks in each table to allowfor compensation of the digital delay lines 202. The compensation couldtake on a form shown in FIG. 4, wherein a N bit binary number controls2^(N) registers containing both the delay and compensation information.The compensation would ensure that both digital delay lines 202 wouldhave equivalent phase coverage over 360°.

In order to produce the necessary vectors, the digital delay lines 202require a reference signal. As amplitude compression is not an issue,the reference can be a high power signal. This high power pulse train200, at the carrier frequency, is supplied to both delay lines. Thedigital delay lines 202 consist of a finite number (N) of sequentialfixed delay cells. The delay of each cell may be equivalent or weighted.Even though the preferred actualization of the invention is to utilizefixed equivalent sequential cells, it could also be implemented using(N) weighted parallel delay cells. The number and weight of the delaycells determine the resolution of the synthesized phase. N should bechosen to realize 3600 coverage with the desired resolution. Thepreferred realization of the invention would also include a finitenumber of extra delay cells which can be used for compensation for thetime resolution steps.

An example of the delay cell implementation is to use an inverter and anedge feedback detector which delays the input pulse by a known amountDelta T. A delayed signal from an output of each delay cell is suppliedto the input of the next delay cell. The delay of the digital delay line202 is set in such a way as to produce the desired phase for the vector.This is accomplished by enabling or disabling specified delay cells inthe delay line. The status of each delay cell is set by the lookup table203. As the delay cell encounters a falling edge it confirms its statuswith the table and has half a pulse cycle to update its status ifrequired.

The pulses exiting 202 a will have the phase that the vector logiccircuit 201 deemed necessary for vector A, while the pulses exiting 202b have the phase deemed necessary for vector B. The pulses then enterthe summer 204, which combines both vectors 302. The resulting vectorhas the phase and amplitude corresponding to the desired modulation.

Since various modifications can be made in my invention as herein abovedescribed, and many apparently widely different embodiments of same madewithin the spirit and scope of the claims without department from suchspirit and scope, it is intended that all matter contained in theaccompanying specification shall be interpreted as illustrative only andnot in a limiting sense.

1. An apparatus for amplitude and phase modulation of a signal comprising: a reference pulse oscillator arranged to provide a signal in the form of a series of input pulses; an input for input modulating data including desired amplitude and phase modulation; a vector logic circuit responsive to the input modulating data; two digital delay lines each coupled to said reference oscillator and each having multiple delay cells for selectively delaying respective pulses of said signal; two lookup tables each of which contains information for controlling the delay cells of a respective one of the delay lines so that the vector logic circuit controls an overall delay of the respective one of the digital delay lines using the information so as to generate therefrom a component vector which is dependent upon the input modulating data; and a summer that is coupled to the two digital delay lines and arranged to combine together the component vectors from both of the delay lines to provide an output vector.
 2. The apparatus according to claim 1 wherein said vector logic circuit is arranged to utilize the desired amplitude and phase modulation to determine the phase of the two fixed magnitude component vectors.
 3. The apparatus according to claim 2 wherein the formula ±Cos⁻¹[r/(2V)] governs the phase offset of the component vectors from the desired output phase, where, in the governing formula, r represents the desired output magnitude and V is the magnitude of the component vectors.
 4. The apparatus according to claim 2 wherein said component vectors are assumed to have the same magnitude and be equidistant, radially, from the resultant vector.
 5. The apparatus according to claim 1 wherein said vector logic circuit is arranged to compensate for the special cases where the phase of leading or trailing vectors crosses the 360° barrier, where compensation is accomplished by either adding or subtracting 2π from the absolute phase of the vector.
 6. The apparatus according to claim 1 wherein said vector logic circuit is arranged to convert said phase modulation into an equivalent delay.
 7. The apparatus according to claim 6 wherein said vector logic circuit is arranged to update the lookup tables with the information required to reproduce the required delay.
 8. The apparatus according to claim 1 wherein said delay lines contain a number of sequential or parallel delay cells capable in combination of covering 360° of phase.
 9. The apparatus according to claim 8 wherein said delay cells have equivalent or weighted delay periods.
 10. The apparatus according to claim 1 wherein said delay lines contain a finite number of additional delay cells for the purpose of compensation in the range of the finest resolution step.
 11. The apparatus according to claim 1 wherein said delay cells contain a feedback edge detector whereupon detection of a falling edge enables the delay cell to confirm its next status from a lookup table.
 12. The apparatus according to claim 1 wherein said lookup tables contain the information required to reproduce a specified delay.
 13. The apparatus according to claim 1 wherein said lookup tables are directly referenced by the digital delay lines in order to control which delay cells are enabled at a given time.
 14. The apparatus according to claim 1 wherein said lookup tables contain redundant registers which contain both delay and compensation information.
 15. The apparatus according to claim 1 wherein said digital delay lines generate component vectors of constant amplitude and wherein the summer is coupled to the two digital delay lines for the purpose of combining the two constant amplitude component vectors into a resultant vector containing a desired amplitude and phase.
 16. The apparatus according to claim 1 wherein said input pulses form a high power pulse train.
 17. The apparatus according to claim 1 in which the input for said input modulating data is digital such that digital data is converted into the output vector which is an analog signal, without the use of digital to analog converters.
 18. The apparatus according to claim 1 in which the input for said input modulating data is digital such that digital data is converted into the output vector which is an analog signal, and wherein the output vector is transmitted with minimal amplification.
 19. The apparatus according to claim 1 in which the input for said input modulating data is digital such that digital data is converted into the output vector which is an analog signal, and wherein the output vector is amplified utilizing non-linear amplifiers. 